Metal and metal alloy for thermoelectric purposes in particular for making thermocouples



Patented Apr. 28 1925.

UNETED stairs Parana @FFHQ WILHELM ROHN, OF HANAU, GERMANY.

METAL AND METAL ALLOY FOR THERMOELECTRIC' PURPOSES IN PARTICULAR FOR MAKING THERMOCO'UPLES.

' No Drawing. Application filed May 14,

To all whom it may canoe m:

Be it known that I, Dr. VVI HELM ROHN, chemist, a citizen of Germany, and resident of Hanan, Germany, have invented new and useful Metals and MetalAlloys fol-Thermoelectric Purposes inP-articular for Making Thermocouples, of which the following is a specification. 1

For accurately measuring high temperaranging from. 600 to 1600 in prac-v tice and science chiefly the platinum-platinum-rhodium thermocouple after Le Chatelier is used. As lon against injurious i as they are protected nlluences, such as metal vapors, silicium, carbon etc., the wires made of these precious metals will bear the temperatures well and will not oxidize. .Anv other great advantage ofthesecouples is the possibility of exchanging them. Both metals, platinum and rhodium. can be produced absolutely chemically pure and therefore" will always display the same thermoelectrical properties. As both metals will remain .unafiected in the air at their melting temperature; as they will neither evaporate and will not react on the crucible material, for producing the platinum-rhodium alloy, it is only necessary to weigh off the two components to the required exact weight and inse them together without any special precautions. Provided the melt is kept liquid sufficiently long for the alloy become uniform, one may be certain toobtain alloys which will behermo-electrically identical to all their predecessors and their successors. With the platinum, only chemically pure metal need be melted and drawn into twire. This affords the great advantage thatthe electric measuring instrument, the mil ivoltmeter, by means of which the thermoforces are measured, need be gaged only once for'the respective temperatures, to be certain, that all previously or-subsequently made platinum-rhodium thermocouples, connected to this millivoltmeter,,will alwaysshow the same temperaturereadings.

Notwithstanding these advantages the platinum-rhodium couple has certain disadvantages. The first is the comparatively 59 low'thermoforce, in, consequence of which very sensitive measuring instruments are required to obtain a sufiiciently-ample and well readable stroke, and such measuring instruments are both expensiveand easily subciently composition and th trically.

1920. Serial No. 381,520.

ject toinjury. A further drawback of the platinum-rhodium couples is their very high price. QIt is true, thatwhen'the couple is destroyed the platinum will remain a certain, very considerable value thereof, but frequently a good part of the metal is lost simultaneously with the destruction of the couple, I Inlview of these drawbacks, there has been for some time a tendency to replace the expensive platinum-rhodium thermocouples of small thermoforce, with thermocouples'made of base metals, which may be used up to'temperatures of 1200 or 1250 at least, which have a high thermoforce and are comparatively cheap. "Base metals and alloys suitable for the purposes have been found and thermocouples made therefrom have been widely used. It was, however, impossible to convert base metals in chemicallypure state, without impurities into the reguline form, and also to produce alloys of such base metalswith such accuracy that charges of different proven ience are sufliidentical as regards their purity and erefore also thermoelec- The:reason for this lies-in the fact that base metals, when melted in the presence of air, furnace gases and the like, will be partly oxidized, or partly burn away and willpartly absorb impurities or react through their oxides with the material oi thecrucibles used. Furthermore alloys of such base metals will when melted in the manner. customary in practice, undergo changes in their composition by the said 90 circumstances acting differently on the various components of the alloy and, also in each individual case. In. consequence thereof it was not heretofore possible to produce in practicebase'metals and their alloys 95 which were thermoelectrically identical, and each time a thermocouple was made of another me1t,-the measuring" instruments had to be scaled afresh or suitable resistances had to be interposed betweenthermocouple 0 and measuring instrument so as to reduce. the stroke of the needle to the same degrees. These drawbacks may be obviated if metals obtained pure by chemical means are electrically'melted at a very low pressure or if alloys are thus made therefrom. It is thereby inade impossible for them to ab sorb impurities or to oxidize; .it is no more r necessary to employ reducing andv purifymg agents which might' affect the'thermoalloys cannot, be' subjected. to I force, andchanges by their components undergoing the injuries of the melting process to a various degree, -My general process of y meltingmetals is ing a plication, Serial Number 363,916,v filed set forth in my co-pendarch 6, 1920. The vacuum melting process, therefore,

. permits the conversion of chemically: pure metals into the reguline state without the1r purity and quality being impaired, andthe vacuum melting process, -furthermore allows remelting any amount of times metals and;

'nace gases for them -afi'ord also another advantage.

production of metal .alloys of such an acou-u .racy in composition that melts from difierent charges. will bepractically identical both as regards their ,pliysical properties and their chemical composition, .and thus also thermoelectrically alike. in vacuo it is impossible for the meta-ls to absorb impurities from the "air ,or the furto burn away, .or be oxidi'zedCand also forftheoxides to'react With' the-Lcrucible material, so that the necessary accuracy is safeguarded. The

alloys without their absorbing impurities and without their changing theirvmechaniA cal, physical and chemical properties. It is therefore possible to remelt analloy, which after the first melt still shows small differk ences from the properties stipulated, once or even several times by adding small quantitles of correctingadditions and. so'obtal'n;

the required thermoelectric properties with the necessary accuracy.

1nicroscop'ical and submicroscopical gas inclusions and po'rosities. Thereforelthfe atmospheric oxygen, to'which the thermocouples are. more or less exposed during use will not .find any opportunity of attacking them, ex ceptfon thesurface of the Wires. In consequcncethereof thedestruction of such couples seems to proceedslower and to pcnetrateless deeply than. Wlth metals and the couples.

cases. It is large reserve stock of thermocouples-of the' 1 same melt, in order to avoid having'to pe'rform'such regaging too frequently. It is' By melting but also in the vWhereas heretofore it was neces- In sho'rt, by em-, 'ploying' the vacuum melting process all alloys.

means.

Thermocouples made of Wires of vacuummelted metals and alloys are in their thermoelectrical properties sufficiently identical from charge to charge, so that th'emeasuri-ng instruments need not be regaged'afresh when it becomes necessary to exchange .This renders unnecessary all makeshifts otherwise. employed in such normore necessary to keep a no morev necessary toflkeep wires from d-ifferent melts separately in stock',-and therefore mistakes with all their serious possible consequences are obviated. l

The following is another. proof of the .importance-of the, improvement thereby obtainedzl It is possible to produce, so to say, artificial 'platinum-platinum-rhodium couples by utilizing certain ternary all oys, which, providedtheproper composition, is maintained (indeed with an allowance limit of will correspond within about 5. degrees'with the platinum-rhodium thermo-' from,

couples within the :",whole range GOO-"12008 not only to the absolute reading form of the thermoforce curve. saryto employ with multiple recording im struments platinum-rhodium couples-only, even when part of the temperatures to be recorded by. the said multiple instrument;

was below 1200 and thus would have ad? niitted-the use of base-metal thermocouples,

'it is now possible to use base metal thermocouples in comb nation with platinum-rhodiuni thermocouples,

the .mstruments of mice being noticed in their records. f-This isa result which would not have been possible without the employ mentof the vacuum melting process.

- hat I' 1aim is:

'1. The method of preparing uniform base .i

metals and base metal alloys for use in thernocouples, WlllCll comprises degasih'cation; by :melting under predetermined reduced predetermined temperatures. A thermocouple composed of p'racti cally degasified base'metals'or base metal pressures and In testimony,=th'at lc'l'a'im the foregoing pas my-invention l have signed my name this 27 day of February 1920. v I i Y DR. WILHELM ROHN.

alloys obtained by the usual technical 

